斯威夫特：型自返回数组 [英] Swift: return Array of type self

问题描述

我想编写一个类功能，它将返回类类型的数组。据我了解，我可以用自我为目标cinstanceType。我的目标是创建一个NSManagedObject与fetchObjects方法的扩展。此方法将比返回NSManagedObject子类的阵列。这里是不编译我的伪code的例子：

I would like to write a class function, which will return an array of the class type. As far as i understood, i can use "Self" for the objective-c "instanceType". My goal is to create an extension for a NSManagedObject with a fetchObjects method. This method will than return an array of NSManagedObject subclasses. Here is a example of my pseudo code which does not compile:

extension NSManagedObject {

    class func fetchObjects(entity: String, context: NSManagedObjectContext, predicate: NSPredicate?, sortDescriptors: NSSortDescriptor[]?) -> Self[] {
        // can't define return type of an array with type Self
        // also var declaration does not work
        var objects : Self[]?

        return objects
    }
}

任何想法我如何定义类型自数组？

Any idea how i can define an array of type Self?

感谢您的帮助！

推荐答案

这是我所使用的类似功能的JIST，请注意，这是对的NSManagedObjectContext 延期而不是 NSManagedObject 。同样的事情也很可能在 NSManagedObject

This is the jist of what I use for a similar function, note that it's an extension on NSManagedObjectContext rather than NSManagedObject. Something similar could probably be done on NSManagedObject

protocol NamedManagedObject {

    class func entityName() -> String;

}

extension NSManagedObjectContext {

    func fetchObjects<T:NSManagedObject where T:NamedManagedObject>(entity:T.Type, predicate:NSPredicate? = nil, sortDescriptors:NSSortDescriptor[]? = nil) -> T[]? {
        let request = NSFetchRequest(entityName: entity.entityName())

        request.predicate = predicate
        request.sortDescriptors = sortDescriptors

        var error:NSError? = nil
        let results = self.executeFetchRequest(request, error: &error) as? T[]

        assert(error == nil)

        return results
    }

}

extension MyObjectClass : NamedManagedObject {
    class func entityName() -> String {
        return "MyObjectClass"
    }
}

然后使用它很简单，只要：

Then using it is as simple as:

let objects = managedObjectContext.fetchObjects(MyObjectClass)

请注意，您也可以实现NamedManagedObject与所有NSManagedObjects：

Note that you can also implement NamedManagedObject for all NSManagedObjects with:

extension NSManagedObject : NamedManagedObject {
    class func entityName() -> String {
        return NSStringFromClass(self)
    }
}

如果您还保证所有的类都明确给出的Objective-C友好的名称：

If you also insure that all your classes are explicitly given Objective-C friendly names:

@objc(MyManagedObject)
class MyManagedObject : NSManagedObject { ... }

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